If you are talking about the voltage regulator for the gauges, 6 volts is a good voltage. I would wire up the dash on the bench to make sure you have everything right before you install it.
You can buy some resistor on Amazon to check the gauges. Tie one side of the resistor to ground a the other side to the sender wire. Don’t leave the gauges reading over full scal for more than a few seconds or they will burn up.
10 ohm - full scale
22 ohm - half scale
70 ohm - empty
These are approximate values that are close enough for what you are doing. Get 5-10 watt resistors.